There is a known camera system in which one of lens units of different kinds is selected and the selected lens unit is detachably attached to a camera body. For instance, in an image processing system described in Japanese Patent Laid-Open Publication No. 06-178178, an imaging device including an image sensor is detachably attached to an information processor including a keyboard, a display, a controller and a power source. At this system, electric power is supplied from the power source to the imaging device.
A camera system described in Japanese Patent Laid-Open Publication No. 2001-066489 comprises a voltage VC memory circuit for storing a check voltage guaranteeing operations of all functions of the system, a voltage detection circuit for detecting a power-supply voltage, a current LA memory circuit for storing a maximum rated current allowed to be supplied to an interchangeable lens, and a camera controlling circuit for controlling the camera. The camera controlling circuit sends a value, which relates to the maximum rated current of the current LA memory circuit, to the interchangeable lens and compares a voltage value detected by the voltage detection circuit with the check voltage of the voltage VC memory circuit of a camera body. On the basis of a comparison result, a condition of battery capacity is displayed and a lens controlling circuit controls a lens drive mechanism within a range concerning the maximum rated current received from the camera body.
In the meantime, as to an interchangeable lens camera described in Japanese Patent Laid-Open Publication No. 2002-072052, a function for monitoring a power-supply voltage is provided in an interchangeable lens. On the basis of a value of the power-supply voltage applied to the lens, a waiting state and an operating state of the lens are changed to save electric power.
In both the inventions described in the above-noted Publication Nos. 06-178178 and 2001-066489, since the camera body monitors the power-supply voltage of the entire system, it is necessary to commonly set a minimum operating voltage of the camera body and minimum voltages capable of operating interchangeable lens groups and interchangeable imaging units. When the minimum operating voltages differ in accordance with lens types and when the minimum operating voltage of the lens is different from that of the camera body, it is impossible to manage the power-supply voltage of the entire system. Further, even if the minimum operating voltage is commonly set, it is necessary to determine a threshold in consideration of, for example, a margin of voltage drop of mount contact points provided between the lens and the camera body, because contact resistance of the contact point changes due to deterioration of the mount contact point with time, wear thereof, change of pressing force thereof, and so forth. There is a possibility that maximum number of shooting to be set by one-time charging reduces.
By combining the invention of the above Publication No. 2002-072052 with the inventions of the above Publication Nos. 06-178178 and 2001-066489, the lens is adapted to be capable of monitoring the power-supply voltage of itself and it is possible to solve the shortcomings of the invention described in the above Publication Nos. 06-178178 and 2001-066489 in that the minimum operating voltages differ in accordance with the lens types and the voltage drop varies at the mount contact point. However, even if the invention of the Publication No. 2002-072052 is applied to the inventions of the Publication Nos. 06-178178 and 2001-066489, since the lens and the camera body independently monitor the power-supply voltage, it is necessary to notify a monitoring result to a user by each of the lens and the camera body. For this reason, the lens and the camera body respectively need a display function and a warning-beep generating function. Consequently, the entire system becomes very redundant. What the user desires to know is the state of the battery capacity of the entire system. When the display is independently performed by each of the lens and the camera body, it is troublesome to confirm the display and there is no advantage for the user.